Liquid crystal displays have found broad applications to watches, calculators, measuring instruments, automobile control panels, word processors, pocket computers, printers, computers, and TV sets. Liquid crystal display systems include a twisted nematic (TN) mode, a supertwisted nematic (STN) mode, a dynamic scattering (DS) mode, a guest-host (GH) mode, and a ferroelectric liquid crystal (FLC) mode. The liquid crystal display driving system has started with a static driving system, developed into a multiplex driving system, and recently further developed from a simple matrix system into an active matrix system.
Liquid crystal materials to be used in these displays are required to satisfy various characteristics in accordance with the display system or driving system. In particular, (1) to show a liquid crystal phase in a low-to-high broad temperature range to be driven in a broad temperature range, (2) to have a low threshold voltage to be driven at a low voltage, and (3) to have a low viscosity and a short response time are important characteristics common to all liquid crystal materials irrespective of the display system or driving system.
For the time being, a liquid crystal compound satisfying all the requirements (1to (3) by itself is not available, and a plurality of liquid crystal compounds must be mixed to provide a liquid crystal composition which will exhibit desired characteristics.
Among a wide variety of liquid crystal compounds heretofore developed, those having a cyclohexane skeleton have been used comparatively widely because they satisfy the requirements (1) to (3) comparatively.
Liquid crystal compounds having a cyclohexane skeleton area characterized by their low viscosity as compared with those having other cyclic structures. However, they are not deemed to be pronouncedly superior to those having other structures as for low-temperature characteristics.
In order to decrease the melting point of a liquid crystal material, it is effective to mix an increased number of compounds. For example, it has been a practice to mix a plurality of such analogues as have the same basic skeleton, composed of rings and a linking group(s), but different numbers of carbon atoms at the terminal group. Nevertheless, it is extremely difficult to reduce the melting point of a liquid crystal material so as to prevent crystallization in a low temperature region even by the above-described method.
It has therefore been demanded to develop a liquid crystal compound which has a cyclohexane skeleton and thereby has a relatively low viscosity, which has a reduced melting point or is hardly crystallized in a low temperature region without adversely affecting other characteristics, and which has improved co-solubility with other liquid crystal compounds.
Various liquid crystal compounds having different liquid crystal phase temperature ranges have been mixed in order to obtain a liquid crystal material satisfying the requirement (1). For example, where it is desired to increase a nematic-isotropic phase transition temperature (hereinafter referred to as T.sub.N-I point), a liquid crystal compound having a high T.sub.N-I point could be added in an increased proportion. However, because such a compound also has a high lower limit for the nematic phase (hereinafter referred to a T.sub.C-N point), the resulting liquid crystal composition necessarily has an increased T.sub.C-N point. As a result, the liquid crystal composition often suffers from crystallization, resulting in a failure of practical use.
Hence, a practically useful liquid crystal composition showing a nematic phase over a low-to-high broad temperature range has generally been prepared by mixing 10 to 20 kinds of liquid crystal compounds selected by experience so as to contain a compound having a low melting point, a compound showing a nematic phase at about room temperature, and a compound having a high T.sub.N-I point. On actually making a choice of liquid crystal compounds, consideration should be given to not only broadening of an operating temperature range but optimization of electro-optic characteristics and viscosity according to the end use. That is, the liquid crystal compounds to be combined together to provide a liquid crystal composition should satisfy various requirements, inclusive of compatibility among themselves, to some extent. Therefore, although a large number of liquid crystal compounds are available, the choice of compounds usable for preparation of a practically useful liquid crystal composition is considerably limited.
For instance, liquid crystal compositions for active matrix driving system, such as TFT or MIM, which are now getting predominant among various liquid crystal displays, are demanded to satisfy not only the above-described requirements (1) to (3) but a fourth requirement for a high voltage halding ratio. Should the liquid crystal composition have a low voltage holding ratio, a so-called flicker phenomenon would take place, in which the luminance of pixels that should have been driven flickers.
In general, in order that a liquid crystal composition may have a high voltage holding ratio, it must be chemically stable against heat or light applied in a device and have a high specific resistivity. As a result of investigations in pursuit of liquid crystal compounds meeting these demands, known compounds having an ester group, a cyano group, a pyrimidine ring or a dioxane ring, which have been employed for TN and STN displays, turned out to be unsuitable for an active matrix driving system because they reduce the voltage holding ratio.
Further, in the active matrix driving system, since the display system is the same as a conventional TN mode, the liquid crystal composition to be used must have positive dielectric anisotropy (.sup..DELTA. .epsilon.) as a whole. However, among conventionally employed liquid crystal compounds having positive dielectric anisotropy (hereinafter referred to as p-type liquid crystal compounds), even those having a relatively high .sup..DELTA. .epsilon. similarly reduce the voltage halding ratio. That is, the compounds shown below were revealed to be unsuited to active matrix driving. ##STR4## wherein R represents an alkyl group.
In order to adjust the .sup..DELTA. .epsilon. of a liquid crystal composition for active matrix driving to a proper positive value, p-type liquid crystal compounds having a fluorine atom or a chlorine atom as a functional group have been used for the present time. Examples of such compounds are shown below. ##STR5## wherein R and R' each independently represent an alkyl group, an alkenyl group or an alkoxylalkyl group.
However, although it is possible to achieve electro-optic characteristics necessary for active matrix driving by using these compounds, it is very difficult to prepare a liquid crystal composition having a sufficiently low T.sub.C-N point because many of liquid crystal compounds useful for active matrix driving have a relatively high T.sub.N-I point.
In order to solve this problem, it has been proposed to reduce the T.sub.C-N point of a liquid crystal composition by adding a plurality of such analogous compounds as have the same skeleton but with the number of carbon atoms at the terminal alkyl group varied from 2 to 7, which are selected from the above-mentioned fluorine-containing p-type liquid crystal compounds. However, this approach does not accomplish reduction in T.sub.C-N point to a considerable degree. Moreover, the viscosity of the compound tends to increase as the number of carbon atoms of the terminal alkyl group increases, resulting in deterioration of the response characteristics, the requirement (3).
Thus, a liquid crystal composition for active matrix driving which fulfills all the characteristics (1) to (4) has not yet been developed. Under these circumstances, the characteristics (2), (3) and (4) take precedence over the characteristic (1), we cannot help using liquid crystal composition for active matrix driving. As for the characteristic (1), we cannot help using liquid crystal compounds whose T.sub.C-N point is not sufficiently low. As an expected result, the currently available liquid crystal compositions are often crystallized in low temperatures.
The liquid crystal compositions used in STN liquid crystal displays are demanded to meet the requirements (1) to (3) and, in addition, (5) to have a high elastic constant ratio (K.sub.33 /K.sub.11) for achieving high contrast. The characteristic (2) (to have a low threshold voltage) is also important for the liquid crystal compositions for STN displays widespread in general-purpose equipment, such as laptop computers.
For reduction of the threshold voltage of a liquid crystal composition, it is effective to increase the dielectric anisotropy .sup..DELTA. .epsilon. or to reduce the elastic constant K according to the following formula: ##EQU1## wherein V.sub.th represents a threshold voltage; k represents a proportionality factor; K represents an elastic constant; and .sup..DELTA. .epsilon. represents a dielectric anisotropy.
Liquid crystal compounds having a very large .sup..DELTA. .epsilon. include, for example, those represented by formula: ##STR6## wherein R represents an alkyl group. Use of a large quantity of such a compound with too large a .sup..DELTA. .epsilon. is liable to raise such problems as an increase in electrical current. This deteriorates reliability on actual use as a liquid crystal display.
A liquid crystal composition having a small elastic constant can be prepared by mixing a mother liquid crystal material comprising a p-type liquid crystal compound, a tricyclic liquid crystal compound of three ring system having a high T.sub.N-I point, a relatively small elastic constant, and a negative .sup..DELTA. .epsilon. (hereinafter referred to as an n-type liquid crystal compound) or a tricyclic p-type liquid crystal compound.
p-Type liquid crystal compounds having a high elastic constant ratio K.sub.33 /K.sub.11 include, for example, compounds represented by formula: ##STR7## wherein R' represents an alkyl group, an alkenyl group or an alkoxylalkyl group; and X represents a hydrogen atom or a fluorine atom.
In an attempt to satisfy the threshold voltage characteristics and the contrast characteristics in STN displays, a mixture comprising the above-mentioned p-type liquid crystal compound and the tricyclic p- or n-type liquid crystal compound tends to be crystallized in a low temperature. Hence, similarly to the case of the active matrix driving system, it has been a practice to add several kinds of analogues having the same skeleton with different carbon atom numbers in the moiety R' or, in cases where R' is an alkenyl group, to add several kinds of analogues in which the position of the double bond differs, to thereby prepare a liquid crystal composition with a reduced T.sub.C-N point.
However, the elastic constant ratio K.sub.33 /K.sub.11 of such analogous compounds largely differs with a difference in the carbon atom number or a difference in the double bond position. As a result, cases are often met with, in which the resulting liquid crystal composition has a reduced elastic constant ratio K.sub.33 /K.sub.11 and thereby reduced contrast. Where the method of adding analogues is followed, there is a limit of possible reduction of T.sub.C-N point, the viscosity increases, and the response time is slow. It would be very difficult to design the composition of a liquid crystal composition while taking these problems into consideration.
Under the present situation, liquid crystal compositions for an STN mode which have a low threshold voltage of about 1.2 V have been prepared. However, not having a sufficiently high elastic constant ratio K.sub.33 /K.sub.11 as mentioned above, these compositions have failed to provide STN liquid crystal displays having low-voltage driving properties and high contrast.
As hereinabove discussed, a liquid crystal composition satisfying the requirement (1) (to have a broad temperature range for the liquid crystal phase) should have a low T.sub.C-N point. Since not a few materials actually undergo crystallization even at a temperature higher than their T.sub.C-N point, a highly reliable liquid crystal display should use a liquid crystal composition which is not crystallized even in a low temperature region so as to eliminate display defects due to changes in environmental temperature all over the display area.
A liquid crystal composition consisting of a plurality of liquid crystal single substances often shows a supercooling phenomenon. Therefore, a T.sub.C-N point of a liquid crystal composition is measured by once cooling to a low temperature sufficient for solidification or transition into a glassy state with liquid nitrogen, etc. (for example, to -70.degree. C.) thereby to crystallize, then gradually increasing the temperature, and, during the temperature rise, measuring a transition temperature from the solid to a nematic phase.
However, in the case of a practical liquid crystal composition comprising 10 to 20 kinds of components, because it is not an eutectic mixture, cases not infrequency occur in which the composition crystallizes even if it is preserved at a temperature higher than the lower limit of the nematic phase as measured by the above-mentioned method. The possible temperature range for driving is virtually narrower than the measured temperature range. It is not a rare case that a liquid crystal composition having a T.sub.C-N point of -70.degree. C. is crystallized at room temperature. While liquid crystal displays installed on automobiles or aircraft are demanded to stably exhibit a nematic phase in a temperature widely ranging from -40.degree. to 110.degree. C., a liquid crystal composition which is not crystallized even in storage at -55.degree. C. has not yet been developed. Some of the liquid crystal compositions practically used in displays installed on automobiles is crystallized in about 1 week in storage at, e.g., -25.degree. C.
It should now be understood that a liquid crystal composition, even having a very low T.sub.C-N point, is not always prevented from crystallization at a temperature above the T.sub.C-N point. Accordingly, what is demanded for a liquid crystal composition that satisfies the requirement (1) to show high reliability is not a low T.sub.C-N point but non-crystallization in a low temperature range.
As discussed above, a liquid crystal composition is prepared by mixing various liquid crystal compounds selected so as to agree with a particular display system or a particular driving system but there are limits of improvements of characteristics that can be achieved only with the liquid crystal compounds currently employed. In particular, many of general-purpose liquid crystal compositions are designed with weight on satisfaction of electro-optic characteristics. On reviewing these general-purpose liquid crystal compositions, particularly those for an active matrix driving system, such as TFT or MIM, and those for STN liquid crystal displays, there are few materials that have a broad temperature range for the liquid crystal phase, are not crystallized in lower temperatures, and are thereby highly reliable.
The general-purpose liquid crystals substantially satisfying the requirements (2) to (5) are regarded as highly reliable and practical liquid crystal materials provided that they are not crystallized for a period of about 1 month even in storage in a relatively low temperature, which varies depending on the end use, though.
However, the temperature of the environment in which the liquid crystal display containing such a reliable liquid crystal composition operates is naturally limited.
For the above-described reasons, there has not yet been obtained such a liquid crystal composition that is not crystallized in a lower temperature and therefore has high reliability while sufficiently satisfying electro-optic characteristics demanded in carrying out various display systems or driving systems.